1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to an alkali metal electrochemical cell having an anode-to-cathode (A/C) capacity ratio specifically balanced to improve cell swelling without appreciably detracting from cathode efficiency. A preferred couple is an alkali metal/solid cathode active material and a most preferred chemistry is a lithium/transition metal oxide couple.
2. Prior Art
Lithium batteries have been successfully used to power implantable medical devices such as pacemakers, cardiac defibrillators, and the like. However, it is known that lithium batteries swell when they reach certain voltages during discharge. In the design of medical devices, this swelling must be accounted for so that enough void space is left for the battery volume change in order to prevent damage to the device circuitry. Consequently, the more swelling the battery experiences, the more void space that must be reserved, which directly impacts the device total volume. In the field of implantable medical devices, a smaller total device volume is desired. Thus, it is desirable to minimize or even eliminate the existence of swelling in lithium batteries, especially those used to power implantable medical devices, to provide a more efficient design and improve the safety characteristics of the implantable device.
A chemistry that provides high discharge capacity and high energy current pulses required for powering cardiac defibrillators and other implantable medical devices is the lithium/silver vanadium oxide (Ag.sub.2 V.sub.4 O.sub.11, SVO) couple. U.S. Pat. No. 5,458,997 to Crespi et al. describes an electrochemical cell having a lithium anode and a silver vanadium oxide cathode, wherein the cell is anode limited. This patent states that in the discharge of a conventionally balanced Li/SVO cell, the resistance increases as a function of time beginning at the second voltage plateau 310 on its discharge curve (FIG. 5 of Crespi et al.). In that respect, Crespi et al. rebalances the cell active components to include less of the lithium and electrolyte materials than would be used in a conventionally balanced cell. The rebalanced cell has the ratio of capacity (Q.sub.-) of the anode divided by the capacity (Q.sub.+) of the cathode between 0.45 and 0.70. This rebalancing allows discharge to occur only through the first voltage plateau and the first ramp to the start of the second voltage plateau, thus avoiding that portion of the discharge curve (the second voltage plateau region) in which resistance buildup occurs in conventionally balanced Li/SVO cells. The disadvantage of this approach is a loss of cell capacity by about 250 mAh (FIG. 6, line 370 vs. FIG. 7, line 385 in Crespi et al.).
Therefore, there is a need for a lithium/transition metal oxide battery that is dischargeable to deliver the high capacity needed for powering implantable medical devices and the like, but which experiences little, if any, swelling during discharge. The cell of the present invention fulfills this need by utilizing both voltage plateaus typical in the discharge of a Li/SVO cell for high discharge capacity while at the same time the effects of cell swelling are minimized through a rebalanced anode-to-cathode (A/C) capacity ratio. Thus, by using both voltage plateaus and both ramps and by rebalancing the cell active components, the cell swelling problem is controlled without detracting from the electrochemical reactions at the electrodes. Rebalancing the active components according to the present invention is also useful for decreasing swelling in a lithium/copper silver vanadium oxide cell as well.